It has been proposed that an image reading unit, such as a scanner, in an image forming apparatus uses a white light-emitting diode (hereinafter “LED”) as a light source for reading images. Document reading methods for such an image reading unit often adopt a control technique that maintains luminous intensity of a white LED at a constant level through constant current control. However, when a white LED emits light continuously or for long periods of time, the temperature of the white LED increases. Even when constant current control is performed, chromaticity (x,y) changes as shown in FIG. 20, and a color of a read image changes.
That is, in a document reading method for an image reading unit using a white LED, when the white LED is driven to illuminate continuously, the temperature of the white LED increases. This characteristic of the white LED causes chromaticity to change in a negative direction. For example, when the image reading unit reads 100 document sheets continuously, there will be a change in color of a read image between the first sheet and the 100th sheet. As an example of a technique for correcting color variations caused by increased temperature, there is a proposed technique in which chromaticity is adjusted in a positive direction by reducing a value of drive current passed through a white LED. With this technique, by reducing LED forward current pulse (hereinafter “IFP”) as shown in FIG. 22, it is possible to increase chromaticity (x,y) and correct variations in color of read images caused by increased temperature.
In the technique of the related art described above, even when chromaticity has been changed in a negative direction due to a change in temperature of a white LED, it is possible to restore the chromaticity to that at normal temperature, by reducing a value of drive current passed through the white LED to adjust the chromaticity in a positive direction. However, since the drive current value IFP for the white LED is reduced, luminous intensity of the white LED decreases as shown in FIG. 21. This may cause a problem in which a read image output from the image reading unit is dimmed.